Session M03: Advances in Optical Atomic Clocks

Resolving the gravitational redshift in a millimetre-scale atomic sample

In this talk I discuss recent progress on the accuracy and precision of state-of-the-art optical atomic clocks. The improved measurement stability of this system enables the resolution of a linear frequency gradient consistent with the gravitational redshift within a single millimetre-scale sample of ultracold strontium. Our result is enabled by improving the fractional frequency measurement uncertainty by more than a factor of 10, now reaching 7.6x10^-21. This heralds a new regime of clock operation necessitating intra-sample corrections for gravitational perturbations. In addition, I discuss the ability to tune the relative strength of the on-site and off-site interactions to achieve a zero density shift at a `magic' lattice depth. This mechanism, together with a large number of atoms, enables the demonstration of the most stable atomic clock while minimizing a key systematic uncertainty related to atomic density. Interactions can also be maximized by driving off-site Wannier-Stark transitions, realizing a ferromagnetic to paramagnetic dynamical phase transition.   

An optical clock based on Ar13+

 Optical atomic clocks are the most precise and accurate measurement devices ever constructed, reaching fractional systematic uncertainties below one part in 10¹⁸. Their exceptional performance opens up a wide range of applications in fundamental science and technology. The extreme properties of highly charged ions (HCI) make them highly sensitive probes for tests of fundamental physical theories. Furthermore, these properties make them significantly less sensitive to some of the leading systematic perturbations that affect state-of-the-art optical clocks, making them exciting candidates for next-generation clocks. The technical challenges that hindered the development of such clocks have now all have been overcome, starting with their extraction from a hot plasma and sympathetic cooling in a linear Paul trap, readout of their internal state via quantum logic spectroscopy, and finally the preparation of the HCI in the ground state of motion of the trap, which allows levels of measurement accuracy to be reached that were previously limited to singly-charged and neutral atoms. Here, we present the first operation of an atomic clock based on an HCI (Ar¹³⁺ in our case) and a full evaluation of systematic frequency shifts. The achieved measurement uncertainty is almost eight orders of magnitude lower than any previous frequency measurements using HCI. Measurements of some key atomic parameters confirm the theoretical predictions of the favorable properties for HCIs for use in clocks. Furthermore, the measured isotope shift between ⁴⁰Ar¹³⁺ and ³⁶Ar¹³⁺ confirms theoretical calculations and comparison to the ¹⁷¹Yb⁺ E3 optical clock places the frequency of this transition among the most precisely measured of all time.

    

Precision measurements with a multiplexed optical lattice clock

The remarkable precision of optical atomic clocks offers sensitivity to new and exotic physics through novel tests of relativity, searches for dark matter, gravitational wave detection, and precision probes for beyond Standard Model particles and forces. While much of optical clock research has focused on improving absolute accuracy, many of the proposed searches for new physics can be performed with relative comparisons between clocks.

In this talk we will present recent experimental results in which we have demonstrated a “multiplexed” strontium optical lattice clock consisting of two or more clocks in one vacuum chamber [1]. In intercomparisons between two spatially separated atom ensembles in the same lattice we observe atom-atom coherence times exceeding 26 s using correlated Ramsey spectroscopy and measure a fractional frequency shift at an imprecision below a part in 10¹⁹. We also realize a miniaturized clock network consisting of 6 atom ensembles, corresponding to 15 unique pairwise clock comparisons performed simultaneously, each at a stability comparable to the previous record for clock comparisons. We will discuss our ongoing campaign of systematics evaluation for a test of the gravitational redshift at the sub-cm scale, and the prospects for future applications of the multiplexed optical lattice clock to searches for dark matter, novel tests of relativity, entanglement-enhanced quantum clocks, and precision isotope shift measurements to hunt for new forces.

 

Four-second optical coherence between different atomic species, and the search for new physics with atomic clocks

The extreme precision and accuracy of today’s optical atomic clocks can be used to look for very small deviations from the predictions of the Standard Model, offering a tool to search for beyond Standard Model (BSM) physics complementary to particle accelerators.  These searches are based on measuring the frequency ratio of two atomic transitions that depend differently on interactions with BSM particles or fields.  In this talk, I will present frequency ratio measurements between atomic clocks based on Al⁺, Hg⁺, Sr, and Yb atoms, and the use of these measurements to constrain the coupling of ultralight scalar dark matter candidates to the Standard Model.  The precision of traditional, incoherent frequency ratio measurements and resulting constraints on BSM physics are limited by the coherence time of the lasers used to probe the atomic transitions.  We have recently demonstrated a new, coherent frequency ratio measurement technique that removes this limitation and achieved a record for the precision of frequency ratio measurements between different atomic species.